Semiconductor optical element

ABSTRACT

A semiconductor optical element has an active layer including quantum dots. The density of quantum dots in the resonator direction in a portion of the active layer in which the density of photons is relatively high is increased relative to the density of quantum dots in a portion of the active layer in which the density of photons is relatively low.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor optical element used asa light source for optical fiber communication or optical disk recordingand a method of manufacturing the same.

2. Background Art

A semiconductor laser having quantum dots formed in its active layer(see, for example, IEEE JOURNAL SELECTED TOPICS IN QUANTUM ELECTRONICS,VOL. 11, No. 5, SEPTEMBER/OCTOBER 2005, pp 1027-1034) and awaveguide-type light receiving element having quantum dots formed in itsabsorption layer (see, for example, IEEE JOURNAL OF QUANTUM ELECTRONICS,VOL. 35, No. 6, JUNE 1999, pp 936-943) has been proposed. Asemiconductor laser in which the density of quantum dots in its activelayer is varied in a plane perpendicular to the resonator direction(light travel direction) has also been proposed (see, for example,Japanese Patent Laid-Open No. 11-307860).

SUMMARY OF THE INVENTION

The distribution of the density of photons in the active layer of asemiconductor laser is such that, as shown in FIG. 17, the density ofphotons is maximized in the vicinity of a phase shift region of adiffraction grating. That is, the semiconductor laser has a photondensity distribution not uniform in the resonator direction in theactive layer. On the other hand, the density of quantum dots in theconventional semiconductor laser is constant in the resonator directionin the active layer, as shown in FIG. 18.

Therefore, electrons and hole carriers in the active layer are reducedby hole burning in a portion of the active layer in which the photondensity is high to cause a local reduction in gain. There is a problemthat the emission efficiency of the semiconductor laser is reduced bysuch a local reduction in gain.

In view of the above-described problem, an object of the presentinvention is to provide a semiconductor optical element in which thelocal reduction in gain due to hole burning is reduced to improve theemission efficiency, and a method of manufacturing the semiconductoroptical element.

According to one aspect of the present invention, a semiconductoroptical element has an active layer including quantum dots, wherein thedensity of the quantum dots in a resonator direction in a portion of theactive layer in which the density of photons is relatively high isincreased relative to the density of the quantum dots in a portion ofthe active layer in which the density of photons is relatively low.

The present invention ensures that the local reduction in gain due tohole burning can be reduced to improve the emission efficiency.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a semiconductor optical elementaccording to a first embodiment of the present invention;

FIG. 2 is a sectional view in the resonator direction of thesemiconductor optical element according to the first embodiment of thepresent invention;

FIG. 3 is a graph showing the distribution of the density of photons inthe resonator direction in the active layer of the semiconductor opticalelement according to the first embodiment of the present invention;

FIG. 4 is a graph showing the distribution of the density of quantumdots in the resonator direction in the active layer of the semiconductoroptical element according to the first embodiment of the presentinvention,

FIG. 5 is a perspective view of a semiconductor optical elementaccording to a second embodiment of the present invention;

FIG. 6 is a graph showing the distribution of the density of photons inthe resonator direction in the active layer of a semiconductor opticalelement according to a third embodiment of the present invention;

FIG. 7 is a graph showing the distribution of the density of quantumdots in the resonator direction in the active layer of the semiconductoroptical element according to the third embodiment of the presentinvention;

FIG. 8 is a graph showing the distribution of the density of photons inthe resonator direction in the active layer of a semiconductor opticalelement according to a fourth embodiment of the present invention;

FIG. 9 is a graph showing the distribution of the density of quantumdots in the resonator direction in the active layer of the semiconductoroptical element according to the fourth embodiment of the presentinvention;

FIG. 10 is a diagram showing a wavelength converter according to a fifthembodiment of the present invention;

FIG. 11 is a graph showing the distribution of the density of photons inthe resonator direction in the absorption layer of a semiconductoroptical element according to a sixth embodiment of the presentinvention;

FIG. 12 is a graph showing the distribution of the density of quantumdots in the resonator direction in the active layer of the semiconductoroptical element according to the sixth embodiment of the presentinvention;

FIG. 13 is a sectional view for explaining a method of manufacturing asemiconductor optical element according to a seventh embodiment of thepresent invention;

FIG. 14 is another sectional view for explaining a method ofmanufacturing the semiconductor optical element according to the seventhembodiment of the present invention;

FIG. 15 is a sectional view for explaining a method of manufacturing asemiconductor optical element according to an eighth embodiment of thepresent invention;

FIG. 16 is another sectional view for explaining a method ofmanufacturing the semiconductor optical element according to the eighthembodiment of the present invention;

FIG. 17 is a graph showing the distribution of the density of photons inthe resonator direction in the active layer of a conventionalsemiconductor optical element; and

FIG. 18 is a graph showing the distribution of the density of quantumdots in the resonator direction in the active layer of the conventionalsemiconductor optical element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a perspective view showing a semiconductor optical elementaccording to a first embodiment of the present invention. FIG. 2 is asectional view of the semiconductor optical element shown in FIG. 1,taken along a resonator direction. The semiconductor optical element inthis embodiment is a ridge-type distributed feedback laser having anactive layer including quantum dots, and a diffraction grating in aresonator.

As illustrated, on an n-GaAs substrate 1 are successively formed ann-GaAs cladding layer 2, an n-AlGaAs diffraction grating 3, an n-GaAsdiffraction grating buried layer 4, an AlGaAs optical confinement layer5, an active layer 6 having InAs quantum dots in AlGaAs, a p-GaAscladding layer 7, a p-GaAs cladding layer 8, and a p-GaAs contact layer9. A ridge is formed of the p-GaAs cladding layer 8 and the p-GaAscontact layer 9, The upper surface of the p-GaAs contact layer 9 and theridge inner walls are covered with SiO₂ insulating film 10. A Ti/Pt/Auelectrode (p-side electrode) 11 is formed so as to be connected to thep-GaAs contact layer 9 through an opening in the SiO₂ insulating film10. A Ti/Pt/Au electrode (n-side electrode) 12 is formed on the backsurface of the n-GaAs substrate 1.

FIG. 3 is a diagram showing the distribution of the density of photonsin the resonator direction in the active layer of the semiconductoroptical element according to the first embodiment of the presentinvention. As shown in FIG. 3, the density of photons is high in thevicinity of a phase shift region 13. Then, the density of quantum dotsis relatively increased in the vicinity of the phase shift region 13 ofthe n-AlGaAs diffraction grating 3, as shown in FIG. 4. That is, thedensity of quantum dots in the resonator direction in a portion of theactive layer in which the density of photons is relatively high isincreased relative to the density of quantum dots in a portion of theactive layer in which the density of photons is relatively low.

Thus, the density of quantum dots capable generating a gain is increasedin a portion in which the density of photons is high, thereby limitingthe reduction in carrier density caused per one quantum dot incomparison with the conventional art. The local reduction in gain due tohole burning is thereby reduced to improve the emission efficiency ofthe semiconductor layer.

Second Embodiment

FIG. 5 is a perspective view showing a semiconductor optical elementaccording to a second embodiment of the present invention. The samecomponents as those shown in FIG. 1 are indicated by the same referencenumerals, and the description for them will not be repeated. Thissemiconductor optical element is a buried hetero-type distributedfeedback laser having an active layer including quantum dots and adiffraction grating in a resonator.

As illustrated, a p-GaAs current blocking layer 14 and an n-GaAs currentblocking layer 15 are formed on opposite sides of the n-AlGaAsdiffraction grating 3, the n-GaAs diffraction grating buried layer 4,the AlGaAs optical confinement layer 5, the active layer 6, and thep-GaAs cladding layer 7. In other respects, the construction is the sameas that in the first embodiment. The same effect as that of the firstembodiment is achieved in this way.

Third Embodiment

A semiconductor optical element according to a third embodiment of thepresent invention is a Fabry-Perot type semiconductor laser having anactive layer including quantum dots, such as that in the firstembodiment.

FIG. 6 is a diagram showing the distribution of the density of photonsin the resonator direction in the active layer of the semiconductoroptical element according to the third embodiment of the presentinvention. As shown in FIG. 6, the light density of photons isrelatively high in the vicinity of the emission end surface. Then, thedensity of quantum dots is relatively increased in the vicinity of theemission end surface, as shown in FIG. 7. That is, the density ofquantum dots in the resonator direction in a portion of the active layerin which the density of photons is relatively high is increased relativeto the density of quantum dots in a portion of the active layer in whichthe density of photons is relatively low.

Thus, the density of quantum dots capable generating a gain is increasedin a portion in which the density of photons is high, thereby limitingthe reduction in carrier density caused per one quantum dot incomparison with the conventional art. The local reduction in gain due tohole burning is thereby reduced to improve the emission efficiency ofthe semiconductor layer.

Fourth Embodiment

A semiconductor optical element according to a fourth embodiment of thepresent invention is a semiconductor optical amplifier having an activelayer including quantum dots, such as that in the first embodiment.

FIG. 8 is a diagram showing the distribution of the density of photonsin the resonator direction in the active layer of the semiconductoroptical element according to the fourth embodiment of the presentinvention. As shown in FIG. 8, the density of photons is relatively highin the vicinity of the light emission end surface. Then, the density ofquantum dots is relatively increased in the vicinity of the emission endsurface, as shown in FIG. 9. That is, the density of quantum dots in theresonator direction in a portion of the active layer in which thedensity of photons is relatively high is increased relative to thedensity of quantum dots in a portion of the active layer in which thedensity of photons is relatively low.

Thus, the density of quantum dots capable generating a gain is increasedin a portion in which the density of photons is high, thereby limitingthe reduction in carrier density caused per one quantum dot incomparison with the conventional art. The local reduction in gain due tohole burning is thereby reduced to improve the light emission efficiencyof the semiconductor layer.

Fifth Embodiment

FIG. 10 shows a wavelength converter according to a fifth embodiment ofthe present invention. This wavelength converter is a Mach-Zehnderinterferometer incorporating the semiconductor optical amplifieraccording to the fourth embodiment of the present invention. Whenmodulation light having a wavelength λ1 and continuous-wave (CW) lighthaving a wavelength λ2 are input to this wavelength converter, the CWlight is phase-modulated with the modulation light by the mutual phasemodulation effect of the semiconductor optical amplifier to be output asintensity modulated light from the output end of the element. In thecase of application of the semiconductor optical amplifier according tothe fourth embodiment of the present invention to this kind ofwavelength converter, no carrier deficiency occurs in the vicinity ofthe emission end of the semiconductor optical amplifier and asufficiently high phase modulation effect is ensured, thus improving theconversion efficiency of the wavelength converter.

Sixth Embodiment

A semiconductor optical element according to a sixth embodiment of thepresent invention is a waveguide-type light receiving element having anabsorption layer including quantum dots.

FIG. 11 is a diagram showing the distribution of the density of photonsin the resonator direction in the absorption layer of the semiconductoroptical element according to the sixth embodiment of the presentinvention. As shown in FIG. 11, the density of photons is relativelyhigh in the vicinity of the light incidence end surface. Then, thedensity of quantum dots is relatively reduced in the vicinity of theincidence end surface having a high light intensity and a high photondensity, as shown in FIG. 12.

The absorption coefficient is thereby reduced in the vicinity of theincidence end surface to avoid disturbance of the guide mode of lightpropagating through the waveguide and to enable light to reach an innerportion of the element. Also, since on the inner side of the element thequantum dot density is high and the light absorption coefficient isincreased, the photoelectric conversion efficiency of the lightreceiving element is improved.

Seventh Embodiment

A method of manufacturing a semiconductor optical element according to aseventh embodiment of the present invention will be described withreference to the drawings. The semiconductor optical element accordingto the first embodiment can be manufactured by using this manufacturingmethod.

First, as shown in FIG. 13, then-GaAs cladding layer 2, then-AlGaAsdiffraction grating 3, the n-GaAs diffraction grating buried layer 4 andthe AlGaAs optical confinement layer 5 are formed on the n-GaAssubstrate 1. Subsequently, the active layer 6 formed of AlGaAs is formedby using a metal-organic chemical vapor deposition (MOCVD) method or amolecular beam epitaxy (MBE) method. If In is added in this formation,quantum dots 6 a formed of InAs are formed in the AlGaAs layer so thatthe distribution density is generally uniform in the resonatordirection.

Subsequently, ZnO film 16 not uniform in film thickness in the resonatordirection is formed on the active layer 6 in which quantum dots 6 a areuniformly formed. In this embodiment, the film thickness of the ZnO film16 on a portion of the active layer in which the density of photons isrelatively high in the resonator direction is reduced relative to thefilm thickness of the ZnO film 16 on a portion of the active layer inwhich the density of photons is relatively low.

Subsequently, a heat treatment at a high temperature is performed todiffuse Zn from the ZnO film 16 into the active layer 6. Part of thequantum dots 6 a are thereby disordered and caused to form a mixedcrystal with the active layer 6 to lose the quantum dot 6 a functionsubstantially completely. The degree of this change is high in theportion corresponding to the increased film thickness of the ZnO film 16and having a higher Zn diffusion density. Thus, impurity diffusion notuniform in the resonator direction is performed on the active layer inwhich quantum dots are uniformly formed to disorder part of the quantumdots and to thereby form a nonuniform density distribution of thequantum dots in the resonator direction in the active layer The ZnO film16 is thereafter removed, as shown in FIG. 14.

Eighth Embodiment

A method of manufacturing a semiconductor optical element according toan eighth embodiment of the present invention will be described withreference to the drawings. The semiconductor optical element accordingto the first embodiment can be manufactured by using this manufacturingmethod.

First, as shown in FIG. 15, then-GaAs cladding layer 2, then-AlGaAsdiffraction grating 3, the n-GaAs diffraction grating buried layer 4 andthe AlGaAs optical confinement layer 5 are formed on the n-GaAssubstrate 1. Subsequently, the active layer 6 formed of AlGaAs is formedby using MOCVD or MBE. If In is added in this formation, quantum dots 6a formed of InAs are formed in the AlGaAs layer so that the distributiondensity is generally uniform in the resonator direction.

Subsequently, a resist 17 not uniform in film thickness in the resonatordirection is formed on the active layer 6 in which quantum dots 6 a areuniformly formed. In this embodiment, the film thickness of the resist17 on a portion of the active layer in which the density of photons isrelatively high in the resonator direction is increased relative to thefilm thickness of the resist 17 on a portion of the active layer inwhich the density of photons is relatively low.

Subsequently, Si ions are implanted in the active layer 6 through theresist 17. A heat treatment at a high temperature is thereafterperformed to disorder part of the quantum dots 6 a and cause the same toform a mixed crystal with the active layer 6 to lose the quantum dot 6 afunction substantially completely. The degree of this change is high inthe portion corresponding to the reduced film thickness of the resist 17and having a higher implanted Si ion density. Thus, ion implantation notuniform in the resonator direction is performed on the active layer inwhich quantum dots are uniformly formed to disorder part of the quantumdots and to thereby form a nonuniform density distribution of thequantum dots in the resonator direction in the active layer. The resist17 is thereafter removed, as shown in FIG. 16.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2006-276837,filed on Oct. 10, 2006 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

1. A semiconductor optical element comprising an active layer includingquantum dots, wherein density of the quantum dots in a resonatordirection in a portion of the active layer in which density of photonsis relatively high is higher relative to the density of the quantum dotsin a portion of the active layer in which the density of photons isrelatively low.
 2. The semiconductor optical element according to claim1, wherein the semiconductor optical element is a distributed feedbacksemiconductor laser further comprising a diffraction grating in aresonator, and the density of the quantum dots is relatively higher inthe vicinity of a phase shift region in the diffraction grating.